1. Field of the Invention
The present invention relates to a method for making a high purity alcohol and more particularly to a method for making high purity ethylene glycol monoethers having ultra-low concentrations of reactive unsaturated impurities, such as aldehydes, enals and enols.
2. Description of the Prior Art
Ethylene glycol monobutyl ether or 2-butoxyethanol (EB) is a well-known commercialized product having many commercial and industrial uses. For example, 2-butoxyethanol is known to be utilized in surface coatings such as inks, paints and varnish, cleaning solvents, automotive fluid additives and electronics to name a few. U.S. consumption of 2-butoxyethanol over the past decade has averaged about 140 thousand metric tons and an average growth rate per year of about 2.6 percent.
The method of manufacture of such ethylene glycol monoethers is well known and published in various patents and publications known to those skilled in the art. Generally, the method for making the ethylene glycol monoethers is an addition chemical reaction and typically comprises the liquid-phase reaction of ethylene oxide with different molecular ratios of the appropriate anhydrous alcohol, such as methyl, ethyl or butyl alcohol, under various conditions of temperature, pressure and catalysts. In all three cases ethylene oxide can proceed to react further to produce the monoethers of diethylene glycol, triethylene glycol and higher glycols. To minimize and control the proportion of these higher molecular weight glycol ethers, an excess of the alcohol is fed to the reactor. Furthermore, depending upon the reaction conditions and impurities in the feedstocks and production process, chemical species having an unsaturated bond, such as an aldehyde, enal, or enol, may be produced. The desired glycol monoether is not formed as a pure compound in the reaction but must be separated from these unreacted raw materials, co-products, and impurities.
Typically, the separation is accomplished by distillation or rectification. These distillation columns generally have a plurality of plates or, if utilizing a packed column, theoretical stages. The excess alcohol feedstock is the removed as a low boiling fraction in the first distillation column. Then, the monoethers of diethylene glycol, triethylene glycol and higher glycols are removed as a high-boiling fraction in the following distillation columns. The product, ethylene glycol monoether, is recovered as a low boiling fraction from the second distillation column. The purity of the product glycol ether can be limited by a plurality of factors, such as, for example, economic and environmental incentive to maximize yield. Impurities can't be reduced by discarding a significant amount of the product glycol ether in the heavy fraction. Likewise, the unreacted alcohol stream is recycled to the reactor instead of being purged or discarded. Thermodynamic and mass transfer limitations inherent to one or more of the distillation columns and their designs also limit the purity of the desired product. Additionally, the boiling points of some impurities are too close to the product glycol ether for economic separation. Another consideration is high temperatures and/or residual oxygen in the distillation columns can initiate thermal or oxidative decomposition and generate additional impurities.
In the case of EB, even after distillation the product may contain up to 5 weight % of unsaturated impurities formed in the manufacturing process; such as n-butyraldehyde, enals such as 2-ethylhexenal, and enols such as 2-ethylhex-2-en-1-ol. These species are difficult to separate from the EB by conventional distillation. Moreover, in applications that require extremely high levels of cleaning and minimal residue, such as electronic or nanotechnology applications, these impurities may interfere with the performance characteristics of EB or even react with the substrate or other components in the cleaning mixture
Accordingly, there is a need for an ethylene glycol monoether, and particularly 2-butoxyethanol product having a high purity, and a low to non-detectable concentration of contaminants having an unsaturated bond such as n-butyraldehyde, 2-ethylhexenal, and 2-ethylhex-2-en-1-ol.